It is a current tendency to utilize pigments as colorants not only in various inks and paints but also in those fields in which clearer colorations are required, for example in color filters and inks for ink jet printing.
In recent years, marked advances have been made in the technology of dispersing such pigments. The method which is used for further rationalization of production processes comprises carrying out dispersion so as to increase the pigment concentration as far as possible. However, there is a fundamental problem that as the pigment concentration increases, the fluidity and dispersion stability decrease.
In addition, in the fields of color filters and ink jet recording, where high levels of fastness are required, it is necessary to use high-grade pigments and reduce the particle diameter to a finer level so that clearer colorations may be accomplished. However, high-grade pigments are by nature poor in dispersion stability as compared with other pigments and, further, as the particle diameter becomes finer, the tendency toward aggregation increases and it becomes difficult to obtain stable dispersions.
On the other hand, carbon black is widely used as a pigment showing a high level of blackness. However, it has problems, namely difficulty in attaining high concentrations thereof and insufficiency in dispersion stability due to its characteristics that its primary particle diameter is very small and it is porous and has a large specific surface area. Recently, attempts have been made to use carbon black also as a light-shielding material for black matrices. It is a problem, however, that it is difficult to obtain highly insulating black matrices from carbon black which is highly conductive.
Therefore, for solving such problems, investigations have so far been made concerning, for example, the treatment (surface treatment/modification) of pigments themselves or the development of pigment dispersants, pigment derivatives or surfactants, each having marked ability to wet (being adsorbed on) pigments and/or be wetted to increase the pigment dispersion stability and/or pigment concentration, and a number of research works are now still going on.
Thus, for example, Japanese Kokai Publication Sho-54-037082 discloses a method of attaining good pigment dispersion stability by utilizing a basic group-containing pigment dispersant for dispersing acidic pigments and thus allowing the pigment dispersant to be adsorbed on the pigment surface through acid-base affinity. However, such a force as acid-base affinity cannot be said to be sufficiently effective in causing the pigment dispersant to be adsorbed on the pigment surface, hence the resulting pigment dispersions can never be applied as such materials for the color filters or the inks for ink jet printing as mentioned above.
There is also available a method of stably dispersing minute pigment particles which comprises using a pigment dispersant in increased amounts. However, when the pigment dispersions obtained by this method are used in pigment dispersion-based resist compositions for color filters or black matrices, a problem arises, namely deterioration in developability.
Further, in Japanese Kokai Publication Hei-09-166869, there is disclosed a method of grafting a dispersant having an aziridine, isocyanate, epoxy or the like functional group onto a carboxyl or the like functional group-containing modification of carbon black through reaction with the carboxyl group. This method, however, cannot yet achieve improvements in fluidity and/or dispersion stability at high concentrations.
Therefore, the applicant first paid attention to a carbodiimide group highly reactive with a carboxyl group, in particular, as means for improving the dispersibility of carbon black by grafting thereof and thus developed and proposed, in Japanese Kokai Publication 2002-201381, a treated carbon black species capable of being maintained in a finely dispersed state very stably as obtained by grafting a dispersant having that group within the molecule onto the surface of carbon black.
Such a treated carbon black species as proposed by the applicant is utilized in inks, paints and the like and, in addition, since the coat layer formed using the same has high levels of insulating and light-shielding properties, it can be used also in resist compositions for producing highly insulating black matrices.
For enabling the formation of coat layers with higher levels of light-shielding and insulating properties, however, there is room for contrivance for improving such fundamental performance characteristics as high carbon black concentrations and dispersion stability. Further, in cases where the range of application is to be extended to the dispersion of organic pigments, there is a limit.